The expression "lie down" refers to the act of assuming a horizontal position on a surface. However, there are several alternatives that can be used to convey the same meaning. Among the most common synonyms are "recline," "rest," "stretch out," "lie flat," "lie back," and "be prone." Each of these words can be used interchangeably with "lie down" in different contexts. For instance, one could say "I need to recline for a moment," or "Why don't you stretch out on the couch?" Depending on the context, these synonyms can add variety and nuance to speech or writing.